1. Field of the Invention
The present invention relates generally to electron beam apparatus.
2. Background
FIG. 1 is a schematic diagram of an electron-optical arrangement for generating an electron beam in a conventional imaging apparatus, and FIG. 2 is a flow chart of a conventional method of generating an electron beam in such an apparatus. For example, the apparatus may comprise an electron beam lithography apparatus, or it may comprise an electron beam inspection apparatus. In the figure, axial ray trajectories are shown as solid lines and field ray trajectories are shown as dashed.
A source 102 is defined to be located at the object plane. The source 102 generates 202 electrons at the object plane. The source 102 may comprise, for example, a conventional electron source, a linear or two-dimensional array of such sources, or an extended area source. Alternatively, it may be an extended source of secondary electrons (such as a wafer) or an aperture illuminated by primary or secondary electrons.
A first round or axisymmetric lens 104 receives the electron beam and focuses 204 it in both x and y dimensions, where the round beam is defined as traveling in the z-direction. The focusing 204 of the first round lens 104 is such that a cross-over point 106 is formed 206 by the round beam. The cross-over point 106 comprises a spot at which the charge density of the round beam becomes very densely concentrated. An aperture may optionally be placed at the field ray cross-over point 106 to limit the numeric aperture of the beam. At sufficiently high beam currents and/or sufficiently small numeric apertures, detrimental space charge defocusing effects occur at and near such a cross-over point 106.
A second round or axisymmetric lens 108 receives the electron beam after the cross-over point 106. The second round lens 108 focuses 208 the round beam in both x and y dimensions, such that an illumination spot 110 is formed 210 at an image plane. The image plane may be defined to be at a surface of a substrate, or at an electron detector.
It is desirable to improve electron beam apparatus and methods or forming incident electron beams. In particular, it is desirable to reduce space charge defocusing in electron beam apparatus for projection microscopy, lithography and other applications.